When is a Triassic fauna not Triassic?

In recent years among paleontologists who work on the Triassic/Jurassic boundary there has been some serious excitement about a new locality in northeastern Utah that hosts a wide variety of cool fossils. It has been named the Saint's and Sinners Quarry and has been actively worked by crews from Brigham Young University in Provo, Utah since 2009. Based on abstracts and news articles it is clear that the fauna is diverse and well represented by multiple specimens. Having been at SVP in recent years I have been able to see images of the fossils coming out of the quarry first hand. Over 11,500 fossils have been removed from the quarry which Brooks Britt (from BYU) and others estimate is only 33% excavated. Virtually all of the fossils are preserved in 3D, allowing us to have spectacular insights into animals we do not have much data from, due to crushing and other concerns. Most of the specimens are even articulated! My hat is off to all of the BYU and Dinosaur National Monument crews who have been literally working on the edge of a cliff to extract these remains.

But. You knew there was a "but" coming, didn't you? But while the fossils themselves are spectacular there has been a trend in the last couple years to refer to this bone bed as being Late Triassic in age.  Admittedly aeolian deposits are hard to date; they tend to lack any significant ash deposits and detritial zircons (which can be used to constrain ages in other sedimentary rocks) are not really useful in sand dunes. That is what the Nugget Sandstone is - a deposit of windblown sand in western North America that began during the latest Triassic Period and persisted well into the Early Jurassic (see Sprinkel et al., 2011 for more details). This sand sea expanded as paleolatitude changed and western North America drifted further away from the equator and into the "dry belt" where warm, arid climatic conditions exist. This pattern can even be seen in the Late Triassic Chinle Formation at Dinosaur National Monument, as presented on at SVP this year (Irmis et al., 2015).

The first reports of the quarry (Chambers et al., 2011) suggested that Britt and colleagues at first assigned an Early Jurassic age to the deposit. This date was keeping with the general consensus that the Triassic/Jurassic boundary was somewhere within the Nugget. By 2012, however, it appeared that the teams views changed. That year Engelmann and others (note -the actual abstract doesn't appear to be available any longer) presented an abstract at the GSA conference in Charlotte, NC. In the title they state that a new drepanosaur has been found in the Nugget Sandstone and state that it has biostratigraphic importance. They also explicitly question the Jurassic age of the Nugget (they literally put a question mark in front of the word Jurassic) based on this new find. This new drepanosaur is pretty dang cool! The team expanded on it in recent SVP meetings (Chure et al., 2013; Chure et al., 2015). This critter seems to show highly derived characters shared only with Drepanosaurus (a European form) that indicate it was a specialized fossorial (digging) animal. The kicker here is that all other known drepanosaurs come from definitive Triassic strata. The Nugget drepanosaur comes from a quarry 55 meters above the last reliably dated strata (the Bell Canyon Formation, which sits between the Chinle and Nugget in northeastern Utah).

So what's the problem? Well this year the team again presented on some more spectacular fossils from the Saints and Sinners Quarry, including a large toothed pterosaur that is very closely related to the Early Jurassic European pterosaur Dimorphodon (Britt et al., 2015). This story has been picked up by the national media who have been reporting this site as being Late Triassic in age. Let's do a quick review of the evidence for a Late Triassic age.

Evidence of a Triassic Age of the Saints and Sinners Quarry

• Presence of a drepanosaur
• Presence of several small sphenosuchians
• In a formation that is traditionally considered to span the Triassic/Jurassic Boundary

Okay...that's not really a convincing list. This is especially true if you are claiming that this extraordinary interdunal wetland deposit represents a Triassic assemblage unlike any other in western North America. In fact two of the "pros" can actually be taken as a "con" and the third I think is ambiguous.
Allow me to present a list of why I have concerns about a Triassic age for this quarry.

Why the Saints and Sinners Quarry may be Jurassic in age

• In a formation that is traditionally considered to span the Triassic/Jurassic Boundary
• Quarry located 55 meters above the last Triassic-dated rocks (~1/2 the thickness of the Nugget)
• Presence of the most-derived drepanosaur yet discovered
• Presence of a pterosaur that is most similar to a Jurassic pterosaur
• Presence of a medium-large bodied theropod in the quarry in addition to a coelophysoid
• Presence of several small sphenosuchians
• No phytosaurs
• No aetosaurs
• No metoposaurs
• Upper Nugget lacks a Triassic ichnofauna

Well, does this mean case closed? No. While my list may be longer it isn't the final word on anything. Several of these points rely on the absence of taxa like phytosaurs and we all know that the absence of evidence is not the evidence of absence. Still, taken as a suite of things, I am not convinced that this quarry is Triassic. There are a few ways that perhaps we could do to see if I'm wrong.

• Phylogenetic analysis of the sphenosuchians - closely related to Chinle or Kayenta taxa?
• Phylogenetic analysis of new drepanosaur compared to the still-unnamed Ghost Ranch form
• Phylogenetic analysis of the theropods - are they closer to Coelophysis or later taxa?
• Additional fieldwork to look for unambiguous biostratigraphic markers

To me this fauna looks like a typical Early Jurassic fauna from western North America with a drepanosaur thrown in. Could it be an impoverished Late Triassic fauna that also has several highly derived taxa in it? I suppose and I will happily eat my hat if that is the case. What a great collection of Triassic taxa it would be! With the data that have been presented thus far I just can't see it though.

Why does this matter? Timing is everything in evolution. One of the big ways we as paleontologists talk about paleobiogeography is in terms of dispersal and vicariance. Are animals (and plants, and fungi, etc.) slowly moving into new areas or are populations split up by new barriers, isolating groups that then adapt in their own directions? To put it in the context of the Nugget fossils, are we seeing evidence that many disparate clades were widespread in the Late Triassic, or are we seeing similar taxa from elsewhere in North America in the Early Jurassic adapting to new environments? These questions have serious implications for our understanding of the rate of evolution among all these groups. By tying down the date of the Saints and Sinners Quarry we will be better able to answer some of these questions.

Final caveat: this is all based off of abstracts, talks, and posters and conferences, some of which I was unable to attend or access (this is why people should archive their conference presentations on FigShare - but I digress). I am extremely excited to see the peer reviewed publications that should result from these finds. And it may very well be that their method for dating the quarry is more nuanced than they have already presented. As always, I suppose, "Wait for the paper."

Works Cited
Britt, B. B., Chure, D., Engelmann, G., Dalla Vecchia, F., Scheetz, R. D., Meek, S., Thelin, C., Chambers, M. A NEW, LARGE, NON-PTERODACTYLOID PTEROSAUR FROM A LATE TRIASSIC INTERDUNAL DESERT ENVIRONMENT WITHIN THE EOLIAN NUGGET SANDSTONE OF NORTHEASTERN UTAH, USA INDICATES EARLY PTEROSAURS WERE ECOLOGICALLY DIVERSE AND GEOGRAPHICALLY WIDESPREAD. Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 97

Chure, D. J., Andrus, A. S., Britt, B. B., Engelmann, G. F., Pritchard, A. C., Scheetz, R., Chambers, M. MICRO CT IMAGERY REVEALS A UNIQUE MANUS MORPHOLOGY WITH DIGGING/SCRATCHING ADAPTATIONS IN THE SAINTS AND SINNERS QUARRY (SSQ) DREPANOSAUR, NUGGET SANDSTONE (LATE TRIASSIC), NORTHEASTERN UT Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 107

Chure, D., Britt, B., Engelmann, G., Andrus, A., Scheetz, R. DREPANOSAURS IN THE DESERT: MULTIPLE SKELETONS OF A NEW DREPANOSAURID FROM THE EOLIAN NUGGET SANDSTONE (?LATE TRIASSIC - EARLY JURASSIC), SAINTS AND SINNERS QUARRY, UTAH: MORPHOLOGY, RELATIONSHIPS, AND BIOSTRATIGRAPHIC IMPLICATIONS Journal of Vertebrate Paleontology, Program and Abstracts, 2013 p. 106

Chambers, Mariah, Hales Kimberly, Brooks B. Britt, Daniel J. Chure, George F. Engelmann, and Rod Scheetz. "Preliminary taphonomic analysis of a Ceolophysoid theropod dinosaur bonebed in the Early Jurassic Nugget Sandstone of Utah." In Geological Society of America Abstracts with Programs, vol. 42, no. 4, p. 16. 2011.

Engelmann, G., Britt, B., Chure, D., Andrus, A., Scheetz, R. MICROVERTEBRATES FROM THE SAINTS AND SINNERS QUARRY (NUGGET SANDSTONE: ?LATE TRIASSIC–EARLY JURASSIC): A REMARKABLE WINDOW ONTO THE DIVERSITY AND PALEOECOLOGY OF SMALL VERTEBRATES IN AN ANCIENT EOLIAN ENVIRONMENT  Journal of Vertebrate Paleontology, Program and Abstracts, 2013 p. 122

Engelmann, George F., Daniel J. Chure, Brooks B. Britt, and Austin Andrus. "The biostratigraphic and paleoecological significance of a new drepanosaur from the Triassic-? Jurassic Nugget Sandstone of northeastern Utah." In 2012 GSA Annual Meeting in Charlotte. 2012.

Irmis, R. B., Chure, D. J., Wiersma, J. P. LATITUDINAL GRADIENTS IN LATE TRIASSIC NONMARINE ECOSYSTEMS: NEW INSIGHTS FROM THE UPPER CHINLE FORMATION OF

NORTHEASTERN UTAH, USA Journal of Vertebrate Paleontology, Program and Abstracts, 2015 p. 149

Sprinkel, Douglas A., Bart J. Kowallis, and Paul H. Jensen. "Correlation and age of the Nugget Sandstone and Glen Canyon Group, Utah." Utah Geological Association Publication 40 (2011): 131-149.

A Toothy Issue

I am going to talk about teeth today. When I first knew I was going to get into paleontology I didn't think I would every really study teeth. I mean, teeth are neat and everything but I wanted to study dinosaurs! Dinosaurs, especially when I was younger, were mainly known for having relatively simple and easily-identifiable teeth that didn't tell us much besides diet. The only people who studied teeth were mammal paleontologists (which I foolishly looked down upon in my middle and high school years).

Even as I progressed through college I didn't pay much attention to teeth. Sure there were some odd teeth known from the Triassic Period, like Revueltosaurus and Tecovasaurus, but they were rare and the exception to the rule. I figured that they provided only marginal information on the ecosystem and that the major components were well known and understood - things like phytosaurs, metoposaurs, aetosaurs, and rare dinosaurs like Coelophysis. Well it turns out, unsurprisingly, that this view is naive and wrong.

Some of this change has come about from the work of Andy Heckert in the early years of this century. Although his treatise on Chinle microvertebrates is somewhat out of date now (it was published by the New Mexico Museum of Natural History and Science in 2004) it helped establish that the diversity of animals living in western North America was much higher during the Triassic Period than people had previously suspected. In addition to naming new taxa like Krzyzanowskisaurus, Protecovasaurus, and Crosbysaurus, his PhD work showed many new tooth types from the Chinle Formation and Dockum Group that had never been reported in the scientific literature!

Our work at Comb Ridge has focused on teeth. This is not because we set out to find lots of teeth. As with most things in paleontology you focus on what you find. At Comb Ridge we haven't found phytosaur skulls and troves of fossil fish like we do further north. We haven't found aetosaurs like in Arizona or mass graves of dinosaurs like in New Mexico. Instead we are finding teeth. Lots and lots of teeth. So many teeth that one locality, The Hills Have Teeth, may be the most productive microfossil site in Utah - it is certainly the most productive microsite in the Chinle of Utah. We have a dozen species represented, possibly more, from this one hill and they are all known from their teeth. So let's have a brief overview of tooth anatomy and terms so that it doesn't seem like I'm speaking gibberish in future posts.

Handy guide for some of the most common tooth terms I made based on an image from Lopez et al. (2015). Scale bar = 1 mm. CC-BY 4.0

List of Dental Anatomical Terms and Definitions

• Apex - the "top" or tip of a tooth; the portion furthest away from the gumline.
• Apical - a directional term, referring to things towards the apex.
• Asymmetrical - a tooth, viewed from the apex, that does not have the same profile on the lip-side as it does on the tongue-side.
• Base - the "bottom" of the tooth; the portion of the tooth at the gumline.
• Basal - a directional term, referring to things towards the gumline.
• Carina - a distinct ridge or edge, usually found along the leading or trailing edge of the tooth.
• Cingulum - a ridge, "waist", or "belt" of thickened enamel running around the tooth near the gumline.
• Circular - refers to a tooth that is circular in outline when viewed from the apex.
• Conical - a tooth that when viewed from the side has a roughly cone-shaped or pyramidal outline.
• Crown - the portion of the tooth from the gumline to the tip. What most people think of when they use the word "tooth."
• Denticles - triangular or angled protrusions along an edge used for cutting food. Can be angled towards the apex or facing perpendicular to the crown height. In some species these can be subdivided into smaller denticles.
• Dentine - the tough inner material that makes up most of a tooth. Very hard but not shiny.
• Distal - the part of the tooth facing the back of the mouth. In older literature this is sometimes referred to as "posterior."
• Enamel - the tough, shiny, outer surface of a tooth. A very hard material!
• Infolding - used to be commonly referred to as "labyrinthodont", which means "maze tooth." These are places on the tooth where the enamel is folded in towards the center of the tooth. It appears wrinkled.
• Labial - the side or portion of the tooth that faces the outside of the mouth. Labial literally means "lips."
• Laterally compressed - refers to a tooth that is much thinner "side to side" than it is "front to back" when viewed from the apex.
• Lingual - the side or portion of the tooth that faces the inside of the mouth. Lingual literally means "tongue."
• Mesial -  - the part of the tooth facing the front of the mouth. In older literature this is sometimes referred to as "anterior."
• Occlusal - the surface, face, or point of the tooth that would rub against ("occlude") the opposite tooth from the opposite jaw. Sometimes used in place of apical when referring to a viewing angle.
• Recurved - a tooth that, when viewed from the side, has the back (distal) side curved inward, so that the edge looks like a half-moon.
• Resorption pit - a pit on the base of a tooth, showing where bone and dentine were reabsorbed by the animal to allow the tooth to be shed.
• Root - in animals with teeth set into sockets, the root is the dentine that extends below the gumline into the jaw to anchor the tooth.
• Serrations - like on a steak knife, these are small notches on the edge of a tooth for cutting or slicing food.

Okay, so there are a number of terms there but I think I've given the definitions in terms that aren't too hard to follow for the average person. Let me show a few examples of teeth so I can sort of show how these terms are used "in the real world."

Crosbysaurus tooth. Scale distance = 1 mm.

The above picture is of part of a Crosbysaurus tooth from one of our sites at Comb Ridge. It shows denticles, the pointed cutting parts on the distal edge (or carina) of the tooth. Each of the pyramid-shaped structures has smaller bumps on them - these are the accessory denticles. This picture is in labial view.

Crosbysaurus tooth. Scale distance = 1 mm.

Here is another view of the same tooth. Here we are looking at the tooth in mesial view with the apex on the right and the base on the left. You can see a resorption pit at the base - it looks like the tooth is hollow. You can notice that this tooth is laterally compressed - it is much narrower than it is tall.

Archosauriform tooth. Scale distance = 1 mm.

Last example. Here is an archosauriform tooth in basal view. The front of the mouth, or mesial side, would be towards the right while the back of the mouth, or distal side, is to the left. You can see in this view that the tooth is asymmetrical - the labial and lingual sides are not equal. This picture also gives a decent view of the resorption pit located in the middle of the base here. That tells us that this is a shed tooth crown.

Thanks for making it through this! I know there were a lot of terms but I promise they will come in handy for many of my future posts. And now you can impress your dentist with your knowledge of dental terminology! The paleontology of teeth (Odontology) is not just for mammal paleontologists. All of this work with microfossils and Triassic teeth has certainly given me a new appreciation of how important these little things can be and what they can tell us about an ecosystem. Just what specifically can they tell us? That sounds like another blog post in its own right.